Photographic media

ABSTRACT

In photographic elements comprising a reversibly activated photoconductor and incorporating a photographic reducing agent, increased shelf life results from using an N-(carboxy-substituted alkyl) aniline compound containing an amino or hydroxy substituent on the aromatic nucleus as the reducing agent.

O United States Patent [151 3,661,587

Adrian 1 May 9, 1972 54] PHOTOGRAPHIC MEDIA OTHER PUBLICATIONS [72]Inventor: Howard Adrian, Sudbury, Mass. llford Manual of Photography,5th Ed., (1958) p. 399 [73] Assignee: Itek Corporation, Lexington, Mass.1322:31 1 ndbok of Photography, McGrawnHm, [22] Filed; APB 1, 9 Clerc.Photography, Theory & Practice, 2nd Ed., Henry [52] U.S. Cl. ..96/76,96/48 PD, 96/66 [51] Int. Cl ..G03c 1/48 [58] Field of Search ..96/29,48, 64, 66, 76

[56] References Cited UNITED STATES PATENTS 2,163,166 6/1939 Wilmanns eta1. ..96/66 3,152,903 10/1964 Shepard et al. ..96/64 3,424,582 1/1969Berman et al ..96/48 Greenwood & Co., Ltd, London 1937) p. 243

Primary E.\'aminer--Norman G. Torchin Assistant E.\aminer.lohn WinkelmanAtt0rney-1-lomer 0. Blair, Robert L. Nathans and W. Gary Goodson [57]ABSTRACT In photographic elements comprising a reversibly activatedphotoconductor and incorporating a photographic reducing agent,increased shelf life results from using an N-(carboxysubstituted alkyl)aniline compound containing an amino or hydroxy substituent on thearomatic nucleus as the reducing agent.

13 Claims, No Drawings PHOTOGRAPHIC MEDIA BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates to an improvedphotographic medium containing a photographic reducing agent in themedium.

2. Description of Prior Art Data or image storage media comprisingradiation-sensitive materials such as titanium dioxide are described indetail in U.S. Pat. Nos. 3,152,903; 3,052,541; French Pat. Nos. 345,206and 1,245,215 and in commonly owned co-pending U.S. application, Ser.No. 199,211, filed May 14, 1962 in the name of Elliot Berrnan et al, nowabandoned. In the aforementioned US. patent application, aradiation-sensitive material, such as titanium dioxide, functions as aphotosensitive component of the media and exposure of said media toactivating means such as radiant energy, electron beams or the likeresults in the storage of a reversible latent image pattern therein. Thereversible latent image pattern exists for a finite time during whichsaid pattern can be converted to an irreversible form and read outvisually by contacting said pattern with a suitable image formingmaterial, such as a chemical redox system. In the aforesaid U.S. andFrench patents, the radiation-sensitive material is combined with atleast one component of an image-forming material prior to exposure toactivating means. For example, U.S. Pat. No. 3,052,541 describesphotosensitive copy media comprising a photosensitive material such astitanium dioxide in combination with a reducible metal ion such assilver nitrate. This copy media is exposed to activating means and thencontacted with a reducing agent to produce a visible image. On the otherhand, U.S. Pat. No. 3,152,903 discloses a system wherein thephotosensitive material is used in combination with both oxidizing agentsuch as silver nitrate and a reducing agent such as hydroquinone. Uponexposure to suitable activating means, a visible image is formed, One ofthe limitations of the above-mentioned data or image storage systems isthat those photo graphic systems containing the reducing agent, e.g.,hydroquinone, in or in contact with the photosensitive medium rapidlyundergo a loss of potency of the reducing agent, presumably due todegradation of the reducing agent while the medium containing same isstored, that is for significant periods of time prior to photoexposureand development. This is a well-known drawback in the use of the commonphotographic reducing agents, hydroquinone and metol. When used, thereducing agents must be stabilized, e.g., with oligosaccharides, andeven when stabilized the maximum permitted shelf-life is only about 6months.

The incorporation of the reducing agent in the photosensitive medium ishighly desirable since it obviates the need of one of the steps ofconventional photographic development, which is obviously quitedesirable particularly in automated processing. Thus, there is a needfor photographic, i.e., photosensitive, media in which the reducingagent is present, with the media retaining a substantial amount of thereducing potential of the reducing agent over prolonged periods ofstorage. Previous attempts at providing such media have, as yet, beenunsuccessful SUMMARY OF THE INVENTION It has now been unexpectedly foundthat certain photographic reducing agents can be incorporated intophotosensitive media, especially those previously described, comprisingat least one photoconductor which becomes reversibly activated uponexposure to an image pattern of activating radiation and thereby capableof causing chemical reaction at portions of said medium corresponding tosaid image pattern and remain substantially stable over prolongedperiods of storage. The reducing agents are N-(carboxy-substitutedalkyl)anilines with an amino or hydroxy substituent on the aromaticnucleus. Additionally the photosensitive medium may comprise at leastone image-forming oxidizing agent.

DESCRIPTION OF PREFERRED EMBODIMENTS The photoconductor or photocatalystis not limited to any group of compounds but may include both organicand inorganic photosensitive materials. Preferred photoconductors usefulin'this invention are metal containing photoconductors. A preferredgroup of such photosensitive materials are the inorganic materials suchas compounds of a metal and a nonmetallic element of group VIA of theperiodic table (*Periodic table from Langes HANDBOOK OF CHEMIS- TRY, 9thEdition, pp. 56-57, 1956) such as metal oxides, such as zinc oxide,titanium dioxide, antimony trioxide, aluminum oxide, zirconium dioxide,germanium dioxide, indium trioxide, hydrated potassium aluminum silicate(K Al Si O H O), tin oxide (SnO bismuth oxide (Bi O lead oxide (P- bO),beryllium oxide (BeO), silicon dioxide (SiO barium titanate (BaTiO),tantalum oxide (Ta O tellurium oxide (TeO and boron oxide (B 0 metalsulfides such as cadmium sulfide (CdS), zinc sulfide (ZnS) and tindisulfide (SnS metal selendides such as cadium selenide (CdSe). Metaloxides are especially preferred metal oxide because of its unexpectedlygood results. Titanium dioxide having an average particle size of about250 millimicrons or less is a preferred photoconductor for thisinvention.

Also useful in this invention as photoconductors are certain fluorescentmaterials. Such materials include, for example, compounds such as silveractivated zinc sulfide and zinc activated zinc oxide Organicphotoconductors suitable for use in this invention are, for example, theimidazolidinones, the imidazolidinethiones, thetetraarylazacyclooctatetraenes, and thiazines, such asl,3-diphenyl-4,5-bis( p-methoxyphenyl)imidazolidinone-2; 4,5-(bis(para-methoxyphenyl)imidazolidinone-2;4-phenyl-5-(para-dimethylaminophenyl)imidazolidinone-2;4,5-bis(para-methoxyphenyl)imidazolidenthione-Z; 3,4,7,'8-tetraphenyll,2,5,6-tetraazacylco-octatetraene-2,4,6,8; and methylene blue.

Also useful as photoconductors in this invention are theheteropolyacidssuch as phosphotungstic acid, phosphosilicic acid, andphosphomolybdic acid.

While the exact mechanism of the present process is not known it isbelieved that the sensitization, i.e., exposure to activating'light,e.g., ultraviolet light, causes the transference of electrons of thephotoconductor from the valence band to the conductance band, or atleast to some similar excited states whereby the electron is looselyheld, thereby converting the photoconductor from an inactive-to anactive form. If the photoconductor in the active form is in the presenceof an electron-accepting agent, a transfer of electrons will take placebetween the photoconductor and the electron-accepting agent and thelatter will be reduced. Accordingly, a simple test to determine whethermaterials have a photoconductor effect is to mix the material inquestion with aqueous silver nitrate. In theabsence of light, little, ifany, reduction of silver ions should occur. At the same time as exposingthe same mixture to light, a control sample of an aqueous silver nitratesolution alone is similarly exposed and if the mixture darkens fasterthan the control sample, the test material is a photoconductor.

The reducing agents contemplated are N-(carboxy-substitutedalkyl)anilines with anamino or hydroxy substituent on the aromaticnucleus. Such compounds include phenylene diamine compounds in which oneof the amino groups contains the said substituent group and aminophenolcompounds with the required N-substituent. The benzene ring of thesecompounds can contain further substituents such as alkyl, e. g., methyl,ethyl, butyl, n-octyl; halogen, e.g., chloro or bromo; alkoxy, e.g.,methoxy or propoxy; or amino, e.g., NH NI-ICl-I N(Cl-l NHEt etc. Inaddition, the carboxy-substituted alkyl group can contain additionalsubstituents, e.g., hydrocarbon radicals, such as methyl, ethyl,cyclohexyl, propyl, phenyl, benzyl, tolyl and the like, withsubstituents such as the carboxy, alkyl, alkoxy, halo and amino.

The particularly preferred reducing agents are those of the formula:

in which R is hydroxy or amino; R is hydrogen or lower alkyl; and Z isan alkylene radical containing up to five carbon atoms in the principalchain between the two valences of the radical; or an acid addition saltthereof.

Of these compounds, the preferred are those in which substituent R ispara to the amino substituent. Exemplary of the preferred compounds are:

N-(p-hydroxyphenyl)glycine N-( p-aminophenyl)glycineN-(p-hydroxyphenyl)-a-alanine N-(p-hydroxyphenyl)-B-alanine which arereadily available or easily prepared. Additional compounds falling withthe ambit of the foregoing description include:

N-(o-aminophenyl)glycine N-(p-aminophenyl)glutamic acidN-(o-hydroxyphenyl )aspartic acid N-( p-aminotolyl)-a-aminobutyric acidN-(p-amino-o-methoxyphenyl)glycine N-(p-hydroxyphenyD-lysine N-(p-hydroxyphenyl)a,a-diaminoppimelic acidN-(p-hydroxyphenyl)-B-phenylalanine Of the foregoing compounds, thepreferred is N-(p-hydroxyphenyl)glycine since it is readily availableand economical.

The reducing agent may be incorporated into the photosensitive medium byany of the art-recognized methods. The reducing agent can be in theemulsion containing the photosensitive material, e.g., thephotoconductor, or in a suitable undercoating or overcoating for thephotosensitive layer. The reducing agent and the photosensitivematerial, e.g., the photoconductor, may be premixed and then applied toa suitable substrate, e.g., film, paper, metal foil, fabric, glass andthe like, using suitable binding materials. Alternatively, thephotosensitive medium may be sprayed with or immersed into a solution ofthe reducing agent and the solvent then removed by drying. In allinstances, it is preferred to obtain a substantially uniformdistribution of the developing agent for maximum efficiency. Forimproved shelf-life, it is preferred to maintain some water in theemulsion layers on the substrate. Accordingly, the drying of the coatingon the substrate can be readily controlled to permit retention of anydesired level of water content. Preferably, the water content iscontrolled at or near the maximum content that can be tolerated by thecoating without sacrificing mechanical properties the are required.

The concentration of the reducing agent on the photosensitive medium isnot critical and can be determined with a minimum of routineexperimentation. As will be appreciated by those skilled in the art, theamount of reducing agent will be determined by the sensitivityrequirements of the developing process used, as well as by therequirements of image density, the degree of photoactivation of thephotosensitive material of the medium and similar such considerations.For most purposes, a concentration of at least about 0.1 percent byweight based on the photosensitive material is usually desirablealthough it is generally preferred to use slightly higherconcentrations, particularly where long shelf-life of the photographicmedia is contemplated, e.g., concentrations of at least about 0.5percent by weight and higher. Of course, should the quantity of reducingagent prove less than necessary, it is always possible to augment thereducing potency by addition of reducing agent during processing.However, to realize the full benefit of the present invention, it ispreferred to utilize sufficient reducing agent in the photographicmedium to meet any eventuality, particularly long shelf-life. This isaccomplished by using excess reducing agent in the formulation of thepresent photographic media. The maximum amount of reducing agent isdictated by the tolerance of the emulsion therefor since excessiveamounts may render the emulsions too difficult to handle in processing.In general, the amount of the reducing agent is dictated by practicalityof handling and the usual dictates of reason with which the skilledartisan is more than familiar. For best overall results, concentrationsin the range of 3 percent to [0 percent by weight based on thephotosensitive material will suffice to meet any eventuality and at thesame time be economically feasible.

The reducing agents of the present invention may also be used inconjunction with auxiliary developing agents which can be incorporatedinto the photographic medium. Since these agents are not the primaryreducing agent, they may be used in smaller concentration than theprincipal reducing agent. Thus, it is possible to utilize smallconcentrations of conventional developing agents such as hydroquinoneand Metol [p-(N-methylamino)phenol] or alternatively pyrazolidonederivatives such as l-phenyl-B-pyrazolidone, lp-tolyl-3-pyrazolidone andother known pyrazolidone derivatives. If desired, these auxiliaryreducing agents may be utilized in the developing process rather thanincorporating them in the photosensitive medium.

The inert carrier sheet of the present new media comprises any suitablebacking of sufficient strength and durability to satisfactorily serve asa reproduction carrier. The carrier sheet may be in any form such as,for example, sheets, ribbons, rolls, etc. The sheet may be made of anysuitable material such as wood, rag content paper, pulp paper, plasticssuch as polyesters, e.g., polyethylene terephthalate, and celluloseacetate, cloth, metallic foil and glass. The preferred form is a thinsheet which is flexible and durable.

It is also useful to use a binder agent to bind the photosensitivematerial and reducing agent to the carrier sheet. In general, thesebinders are translucent or transparent so as not to interfere withtransmission of light therethrough. Preferred binder materials areorganic materials suchas resins, e.g., butadiene-styrene copolymers,poly(alkyl-acrylates) such as poly(methylacrylates), polyamides,polyvinylacetate, polyvinyl alcohol and polyvinylpyrrolidone. When thephotographic media of this invention are used to record information,e.g., images, the photoconductor should be deactivated, conveniently byconditioning in the dark before photoexposure. Such conditioning isgenerally conducted from I to 24 hours. After deactivation, thephotoconductor is not exposed to activating light prior to its exposurefor recording an image pattern.

The period of exposure will depend on the intensity of the light source,the particular imaging material, the photoconductor, the type and amountof catalyst, if any, and like factors known to the art. In general,however, the exposure may vary from about 0.001 second to severalminutes.

The photographic media of this invention can be used to produce positiveor negative prints as described in the aforesaid copending application,Ser. No. 199,21 1.

The photographic media of this invention are especially sensitive toultraviolet light but the spectral sensitivity may be altered byart-recognized procedures. For example, the spectral sensitivity isextended into the visible region of the spectrum by inclusion of dyes,such as cyanine and hemicyanine dyes as described in commonly assignedcopending application, Ser. No. 633,689 filed Apr. 26, l967, or bydoping of the photoconductor with metal ions, such as chromium, tungstenand molybdenum.

After the photographic media of this invention are photoexposed, avisible image is obtained by contacting the exposed media withimage-forming materials such as described in the aforesaid copendingapplication, Ser. No. 199,2ll and in US. Pat. 3,152,903. Theseimage-forming materials include oxidizing agents and are, in general,the oxidizing component of what are generally referred to in the art asphysical developers. The oxidizing agent is generally the image-formingcomponent of the image-forming material. Either organic or inorganicoxidizing agents may be employed and preferably include the reduciblemetal ions having at least the oxidizing power of cupric ion such assilver, mercuric, plumbic, auric, platinum, nickelous, stannous,plumbous, cuprous and cupric ions. Other suitable oxidizing agents arepermanganate ion, various leuco dye materials such as disclosed incommonly assigned copending application, Ser. No. 623,534, and the like.Organic oxidizing agents include tetrazolium salts, such as tetrazoliumblue and red diphenyl carbazone and genarcyl red 68 (methine dye). Aspreviously mentioned, where desired the exposed media may be contactedwith additional reducing agents which supplement the reducing actionbuilt into the media of this invention, either together with theoxidizing agent, or in a subsequent treatment.

The image-forming materials or oxidizing agent may contain organic acidswhich can react with metal ions to form complex metal anions. Further,these agents may contain other complexing agents and'the like to improveimage formation and other properties found to be desirable in this art.

Additional stabilizing and fixing steps such as known to the art mayalso be added to the processes of this invention in order to increasethe life and permanence of the final print.

The following examples are given to illustrate this invention.

EXAMPLE 1 This example illustrates the stability of the reducing agentin the photographic media of this invention.

Two grams of N-(phydroxyphenyl)glycine is added to 238 g. of a slurry oftitanium dioxide and polyvinyl alcohol (in a ratio of 4:l) in water 19percent solids). The resulting mixture is hand coated on paper sheets(Sample A) and the sheets are air-dried. A second series of sheets(Sample B) is prepared in identical manner incorporating glycerine alongwith the reducing agent. Samples of each of the Sample A and Sample Bvariations are periodically exposed to light using a Mervap light sourceof seconds exposure and then processed to obtain a visible image, thedensity of which is measured on a standard densitometer. The processingsolutions are:

Sensitizer: 55 mg. of silver nitrate/liter of distilled water. Fixer:benzotriazole (4 g.) sodium sulfite (2 g.) methanol (100 ml.) distilledwater (550 ml,)

The exposed media containing the reducing agent are immersed in thesensitizer for l0 seconds, in fixer for 20 seconds and a water wash forseconds. Fresh solutions are used for each of the periodicdeterminations.

The results are summarized in the Table l which follows:

Control samples of the paper without developer but processed identicallyshowed a density of zero.

The overall relative speeds of the tested samples decrease with time butthe reducing potency does remain significant particularly in the sampleswhich lacked glycerine. The D max for similar tested samples prepared bycontrolled automated coating on paper followed by controlled heating todry the coated sheets are superior to those obtained by handpreparation.

The reducing potency of similar sheets containing Metol(pmethylaminophenol) tested under identical conditions is zero after 1week as evidenced by failure to obtain a visible image on sensitizingwith aqueous silver nitrate.

EXAMPLE 2 The procedure of Example 1 is repeated except the amount ofreducing agent is 6 grams instead of 2 grams with similar results.

EXAMPLE 3 EXAMPLE 4 Double-weight baryta paper is coated with anemulsion of the following constituents:

10 g. titanium dioxide 4 g. N-(p-hydroxyphenyl)glycine 0.3 g. commercialwetting agent 10.0 g. gelatin 1.3 g. glycerine 2.0 g. 3.7% formalin (pHof the emulsion 7.8)

Samples of the coated paper are periodically exposed for 30 seconds in aRexo contact printer, then sensitized by dipping into aqueous silvernitrate for 0.5 second, and fixed by immersion in aqueous sodiumthiosulfate (40 g./l.) followed by water washing. The image densitiesare then determined. The results are shown in Table 2.

TABLE 2 Time (weeks) Dmax. l 1.01 2 0.93 3 0.93 4 0.89

EXAMPLE 5 An accelerated test to simulate periods of storage commonlyemployed with silver halide emulsion films is used to determine thestability of the present media. Coated paper samples'prepared by theprocedure of Example 4 are heated for 3 days at F. and 62 percent"relative humidity (simulating one year storage'for silver halideemulsion film). The samples are then exposed, sensitized, fixed andwashed as in Example 4 and the image densities determined. These samplesare compared with identical paper samples not subjected to theaccelerated test but otherwise identically exposed, sensitized, fixedand washed. The results show that there is no appreciable difference inthe Dmax of the developed images of the respective samples.

Baryta paper coated with an emulsion containing 4.2 of N-(p-hydroxyphenyl)glycine and 10 g. of titanium dioxide (emulsion weight200 g.) is subjected to the same accelerated test and the reducingpotency retained is adequate by photographic standards.

EXAMPLE 6 The procedure of Example 4 is repeated using Lee Schoellerpaper in lieu of the double-weight baryta paper with similar results.

For example, the paper is coated with an emulsion containing 2.0 g.N-(p-hydroxyphenyl)glycine and 10 g. of titanium dioxide (total emulsionweight 200 g.). After 1 week, the coated paper is exposed, sensitized,fixed and washed as in Example 4 and the Dmax of the visible image is1.36.

The image-forming materials can also be incorporated into the media ofthe present invention prior tophotoexposure and the visible imageobtained after exposure by immersion in water, or by applying water tothe medium by art-recognized techniques, e.g., by spraying. Onphotoexposure, a medium containing the image-forming material and thereducing agent will become at least partially developed since theoxidationreduction reaction can take place in the medium itself prior towater-wetting.

The methods for incorporating the image-forming materials into thepresent media are generally known to those skilled in the art and havebeen described in the literature, e.g., in, U.S. Pat. No. 3,152,903. Forexample using silver ion as an example, silver nitrate in solution canbe applied to the present media in the same general manner as thereducing agent of this invention, preferably in the binder prior tocoating the substrate as hereinbefore described.

EXAMPLE 7 Double weight baryta paper is coated with the emulsiondescribed in Example 4 to which is added 4.0 g of silver nitrate.

Photoexposure of the coated paper followed by water spraying gives avisible image of good density.

What is claimed is:

1. A photographic medium comprising:

a. at least one photoconductor which becomes reversibly activated uponexposure to an image pattern of activating radiation and thereby capableof causing chemical reaction at portions of said medium corresponding tosaid image pattern; and

b. as photographic reducing agent, an N-[carboxy-substitutedalkyllaniline with an amino or hydroxy substituent on the aromaticnucleus.

2. A photographic medium comprising:

a. at least one photoconductor which becomes reversibly activated uponexposure to an image pattern of activating radiation and thereby capableof causing chemical reaction at portions of said medium corresponding tosaid image pattern; and;

b. a photographic reducing agent of the formula in which R is hydroxy oramino;

R is hydrogen or lower alkyl;

and,

Z is an alkylene radical containing up to five carbon atoms in theprincipal chain between the two valences of the radical;

or an acid addition salt thereof.

3. A photographic medium comprising:

a. at least one photoconductor which becomes reversibly activated uponexposure to an image pattern of activating radiation and thereby capableof causing chemical reaction at portions of said medium corresponding tosaid image pattern;

b. a photographic reducing agent of the formula in which R is hydroxy oramino; R is hydrogen or lower alkyl; and,

Z is an alk]ylene radical containing up to five carbon atoms in teprmcipal chain between the two valences of the radical; or an acidaddition salt thereof.

4. Medium as in claim 3 wherein the reducing agent is N-(phydroxyphenyl)glycin'e.

5. Medium as in claim 3 wherein the reducing agent is N-( phydroxyphenyl)-a-alanine.

6. Medium as in claim 1 wherein the photoconductor is an inorganiccompound formed between a metal and a nonmetallic element of Group VlAof the Periodic Table.

7. Medium as in claim 3 wherein the photoconductor is a semiconductivemetal oxide or metal sulfide.

8. Medium as in claim 3 wherein the photoconductor is titanium dioxide.

9. Medium as in claim 3 wherein the photoconductor is titanium dioxideof an average particle size of about 250 millimicrons or less.

10. Medium as in claim 3 wherein the photoconductor is titanium dioxideof an average particle size of about 250 millimicrons or less and thereducing agent is N-(p-hydroxyphenyl)glycine.

l l. A photographic medium comprising at least one photoconductor whichbecomes reversibly activated upon exposure to an image pattern ofactivating radiation and thereby capable of causing chemical reaction atportions of said medium corresponding to said image pattern; at leastone image,- forming oxidizing agent; and a reducing agent therefor,characterized in that the reducing agent is an Nicarboxy-substitutedalkyl) aniline with an amino or hydroxy substituent on the aromaticnucleus.

12. Medium as in claim 11 wherein the reducing agent is N-(p-hydroxyphenyl)glycine.

13. Medium as in claim 12 wherein the image-forming agent is silver ion.

2. A photographic medium comprising: a. at least one photoconductorwhich becomes reversibly activated upon exposure to an image pattern ofactivating radiation and thereby capable of causing chemical reaction atportions of said medium corresponding to said image pattern; and; b. aphotographic reducing agent of the formula in which R is hydroxy oramino; R1 is hydrogen or lower alkyl; and, Z is an alkylene radicalcontaining up to five carbon atoms in the principal chain between thetwo valences of the radical; or an acid addition salt thereof.
 3. Aphotographic medium comprising: a. at least one photoconductor whichbecomes reversibly activated upon exposure to an image pattern ofactivating radiation and thereby capable of causing chemical reaction atportions of said medium corresponding to said image pattern; b. aphotographic reducing agent of the formula in which R is hydroxy oramino; R1 is hydrogen or lower alkyl; and, Z is an alkylene radicalcontaining up to five carbon atoms in the principal chain between thetwo valences of the radical; or an acid addition salt thereof.
 4. Mediumas in claim 3 wherein the reducing agent is N-(p-hydroxyphenyl)glycine.5. Medium as in claim 3 wherein the reducing agent isN-(p-hydroxyphenyl)- Alpha -alanine.
 6. Medium as in claim 1 wherein thephotoconductor is an inorganic compound formed between a metal and anon-metallic element of Group VIA of the Periodic Table.
 7. Medium as inclaim 3 wherein the photoconductor is a semiconductive metal oxide ormetal sulfide.
 8. Medium as in claim 3 wherein the photoconductor istitanium dioxide.
 9. Medium as in claim 3 wherein the photoconductor istitanium dioxide of an average particle size of about 250 millimicronsOr less.
 10. Medium as in claim 3 wherein the photoconductor is titaniumdioxide of an average particle size of about 250 millimicrons or lessand the reducing agent is N-(p-hydroxyphenyl)glycine.
 11. A photographicmedium comprising at least one photoconductor which becomes reversiblyactivated upon exposure to an image pattern of activating radiation andthereby capable of causing chemical reaction at portions of said mediumcorresponding to said image pattern; at least one image-formingoxidizing agent; and a reducing agent therefor, characterized in thatthe reducing agent is an N-(carboxy-substituted alkyl) aniline with anamino or hydroxy substituent on the aromatic nucleus.
 12. Medium as inclaim 11 wherein the reducing agent is N-(p-hydroxyphenyl)glycine. 13.Medium as in claim 12 wherein the image-forming agent is silver ion.